In olefinic polymerization reactions, it is very important that the recycled solvents employed be free of water. This is particularly true in the production of butyl rubber.
Several different methods have been used to dry the solvents and reactants. One particularly useful method is to pass the solvents and monomers through a molecular sieve, such as a Zeolite bed to remove the water present. See Sheivdaev, A. G., et al., "Results of the Industrial Testing of Zeolites in the Production of Butyl Rubber," Absorbenty, Ikh Poluch., Svoistva Primen., Tr. Vses. Soveshch. Adsorbentam, 3rd, 212-15(1971).
While these molecular sieves are very effective in reducing the water present in the recycle stream, it was found that after extended use, an oil deposit forms in the bed. This deposit forms from small amounts of olefins and diolefins present in the solvent stream, causing a loss of these olefin valves. In addition, if these deposits continue to form, they begin to interfere with the capacity of the molecular seive to remove water from the solvent. Regeneration of the molecular sieve using conventional methods (e.g. heating the material at 250.degree. C. for 8 to 16 hours) does not eliminate the problem. In fact, it has been observed that the loss of olefins from the solvent is greater for regenerated molecular sieves than it is for fresh sieves.
It is believed that some acidic sites have been formed on the sieve from decomposition of some of the methyl chloride solvent, either during the dehydration of the solvent or while the sieve is heated to desorb the water. These acidic sites then catalyze polymerization of the monomers present in the recycle stream. This belief is supported by analysis of the residue found in the beds, which showed that the residue contained mostly isobutylene trimer.
While fresh zeolite can be used each time, this alternative is not cost effective. Therefore, a need exists to purge regenerated molecular sieves of the solvent decomposition products.